Positron emission tomography (PET) is a diagnostic imaging technique for measuring the metabolic activity of cells in the human body. PET can show images of blood flow, glucose metabolism in the brain, or rapid changes in activity in various areas of the body. It can be used to show changes in physiology before any change in gross anatomy has occurred. PET has been used in diagnosing diseases such as cancer, heart disease, Alzheimer's disease, Parkinson's disease, and schizophrenia.
PET uses chemical compounds that are labeled with radioactive atoms that decay by emitting positrons. The most commonly used PET radioisotopes are 11C, 13N, 15O, and 18F. Typically, the labeled compound is a natural substrate, substrate analog, or drug that is labeled with a radioisotope without altering the compound's chemical or biological properties. After injection into the tissue, the radiolabeled compound should follow the normal metabolic pathway of its unlabeled counterpart. The labeled compound emits positrons as it moves through the tissue. Collisions between the positrons and electrons that are present in the tissue emit gamma rays that are detectable by a PET scanner.
Radiolabeled thymidine is a PET tracer that is useful for imaging tumors. In particular, 3′-Deoxy-3′-[18F]-fluoro-thymidine (18F-FLT) has been used for visualizing DNA replication in humans and animals. 18F-FLT is incorporated into DNA during the synthesis phase of the cell cycle and therefore is a useful indicator of cellular proliferation.
After injection into a patient, 18F-FLT is taken up by cells and undergoes phosphorylation by thymidine kinase-1 (TK), an enzyme that is expressed during cellular DNA synthesis. The phosphorylated FLT molecule is retained within the cell, which results in its accumulation. As a result, 18F-FLT provides insight into cellular activity and is an excellent proliferation marker for PET tumor studies.
The usefulness of 18F-FLT as a tumor imaging agent has resulted in a need to develop methods for its quick and efficient synthesis. Typical methods for preparing 18F-FLT have low reaction yields. Newer methods that increase yield require more synthetic transformations. Often, these newer methods result in chromophoric byproducts that are produced during the synthesis. For instance, dimethoxy trityl (DMT) is a protecting group that is often used to protect the 5′-hydroxy. During deprotection, DMT-OH and DMT cation are generated. DMT cation has an orange color. The formation of DMT cation is a typical example of chromophoric byproducts that can be produced during the deprotection step.
The presence of chromophoric byproducts complicates the purification process and makes it more difficult and expensive to efficiently produce 18F-FLT. During purification, the radiolabeled product is typically loaded onto a reverse phase column and eluted. If there is a large amount of byproducts, the byproducts can bleed into the final product producing an impure final product.